HorizontalHeaderView: improve documentation

The docs for HorizontalHeaderView (and VerticalHeaderView) is currently at a minimum. We should at least add a small code snippet that shows how to use it together with a TableView. And perhaps even a screenshot? There is currently also almost nothing i the TableView docs that tells you about HeaderViews existence.

Also document how you can change the label inside the header. E.g, show that you can do

model: ["column 1", "column 2", "column 3"]